The research proposed here is directed toward developing a better understanding of how critical features of communication signals are coded and processed by neural circuits in the brain. The specific goal is to elucidate neurobiological processes which are fundamental to natural auditory communication. Auditory nervous systems have the task of processing important timing cues in communication signals, whether in human speech or in the acoustic communication of a diverse array of other vertebrate animals. The basic organization of auditory pathways is similar across vertebrate classes, and certain species of vertebrate animals are particularly valuable for addressing issues relating to the neurobiology of hearing, communication, and the learning processes which they subsume. In particular, the neurophysiological studies proposed here address problems in fundamental neuroscience which are potentially relevant to import mental health issues, including the design of treatments for individuals with auditory-related learning dysfunctions. Recent studies suggest that poor discrimination of auditory time cues by language impaired children may lie at the root of a broad spectrum of learning disorders. The proposed research focuses on the neural processing of time features in the auditory system of a vertebrate animal which uses a limited set of simple sounds for courtship communication. By conducting neuroanatomical and neurophysiological experiments in the auditory brainstem of Pollimyrus (ie., auditory nerve to midbrain), the substrate and method for the coding of temporal features in sonic stimuli will be examined. New knowledge about how the normal vertebrate nervous system responds to these signals will contribute to the basic biomedical science needed to design informed treatments of dysfunction. The fish (Pollimyrus) is an excellent animal model for this research because: 1. Its repertoire of sounds has been carefully characterized, 2. The dominant features of its sounds are temporal patterns, and 3. Its ear appears to be adapted time analysis (the animal~s hearing is based on an array of hair cells like that found in the human ear, but it does not posses a mechanical frequency analyzer like in the mammalian cochlea). This animal has also proven to be outstanding for single neuron physiological studies and for anatomical studies of the auditory pathway. The proposed research combines neurophysiological studies of the auditory system with neuroanatomical pathway tracing from those auditory centers which have been characterized physiologically. Having made a careful choice of an appropriate animal model, I expect that progress will be made toward a better understanding of fundamental mechanisms of vertebrate communication.